1. Field of the Invention
The present invention relates to an optical transmitter and an optical transmission system that transmit an optical signal of a phase modulation format. In particular, it relates to an optical transmitter corresponding to a modulation format in which light whose phase is modulated according to data is intensity modulated in accordance with a clock to generate RZ pulses, and an optical transmission system that transmits a wavelength division multiplexed (WDM) light using a plurality of these optical transmitters.
2. Description of the Related Art
In long distance WDM transmission, as a format for modulating the optical signal transmitted, an RZ-DPSK (Differential Phase Shift Keying) modulation format or an RZ-DQPSK (Differential Quadrature Phase Shift Keying) modulation format has superior characteristics in regards to receiver sensitivity and the like than a conventionally used RZ (Return to Zero) modulation format.
In the above-described RZ-DPSK or RZ-DQPSK modulation format, at first a data signal is superimposed on light by phase modulation, and is then intensity modulated by a clock signal (or a signal divided from a clock) to make an RZ signal.
FIG. 11 is a structural example of a conventional optical transmitter in which an RZ-DPSK modulation format is used. In the conventional optical transmitter, a continuous light CW output from a light source 11, whose wavelength and output level are variable, is DPSK modulated by a phase modulator 12 according to data, and is then converted to RZ pulses by an intensity modulator 13 in accordance with a clock, and thereby an optical signal of an RZ-DPSK modulation format is output. A drive signal corresponding to data from a driver circuit 12A and a control signal from a bias stabilizing circuit 12B are applied to the phase modulator 12. Furthermore, a drive signal corresponding to the clock from a driver circuit 13A and a control signal from a bias stabilizing circuit 13B are applied to the intensity modulator 13. A multiplexer circuit (MUX) 14 multiplexes a plurality of data signals supplied externally to generate a high bit rate data signal DATA, and also generates a clock signal CLK having a frequency corresponding to the bit rate of the data signal DATA. A precoder 15 performs an encoding process in which differential information between the current symbol and the symbol one bit prior is reflected, using the data signal DATA from the multiplexer circuit 14, generates a modulation signal Q according to the data and an inverted signal Q′, outputs them to the driver circuit 12A, and also outputs a clock signal CLK (or a signal divided from a clock signal CLK) synchronized with the signals Q and Q′ to the driver circuit 13A. The bias stabilizing circuits 12B and 13B are circuits for compensating for the drift of operating points due to temperature change or the like by optimizing DC biases applied to the phase modulator 12 and the intensity modulator 13.
In the above-described conventional optical transmitter, a modulation process in which the output light from the phase modulator 12 is made into an RZ signal by the intensity modulator 13 is designated a pulse carver. A well known pulse carver has three methods, wherein the intensity modulator 13 has different operating points as shown in FIG. 12 for example, and the duty cycle of the RZ signal after being carved is known to be different for each method as shown for example in FIG. 13 (for example, refer to A. H. Gnauck, P. J. Winzer, “Optical Phase-Shift-Keyed Transmission”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 1, JANUARY 2005, pp. 115-130).
To be specific, in a method (refer to arrow A of FIG. 12) in which pulse carving is performed by a clock signal CLK between an adjacent trough and crest in the photoelectric response characteristics of the intensity modulator 13, the duty cycle of the RZ signal is 50% (refer to FIG. 13 (A)). Furthermore, in a method (refer to arrow B of FIG. 12) in which pulse carving is performed by a clock signal CLK between adjacent troughs in the photoelectric response characteristics of the intensity modulator 13, the duty cycle of the RZ signal is 33% (refer to FIG. 13(B)). Moreover, in a method (refer to arrow C of FIG. 12) in which pulse carving is performed by a divided clock signal between adjacent crests in the photoelectric response characteristics of the intensity modulator 13, the duty cycle of the RZ signal is 67% (refer to FIG. 13 (C)). On the other hand, as the duty cycle of an RZ signal for which excellent transmission characteristics can be obtained, approximately 65% is generally known to be the most suitable.
The duty cycle in the present specification means the value of the ratio of the pulse width on the high level side with respect to one cycle of a signal waveform, expressed as a percentage. In the case of an optical signal, it is the value of the ratio of the pulse width (pulse width at a level where the intensity of the light pulse is 3 dB lower than its peak) on the emission side with respect to one cycle of a light pulse, expressed as a percentage.
Incidentally, in an optical transmitter in which an intensity modulator as described above is used, there is a case in which a phenomenon (wavelength chirp) occurs whereby the wavelength of a light fluctuates at the time of modulation. The wavelength chirp becomes the main cause of waveform deterioration due to an SPM-GVD effect caused by self phase modulation (SPM) and group velocity dispersion (GVD) occurring in a transmission path. In order to limit waveform deterioration due to the SPM-GVD effect, there are desirable chirp characteristics in optical transmitters used in long distance WDM transmission. Normally, chirp characteristics in which an α parameter indicating wavelength chirp generated, is greater than or equal to 0.7 to 1 is the most suitable.
However, in the aforementioned pulse carver, the α parameter in the case where the duty cycle of the RZ signal is 50% is 1 or −1. Here, α=1 or −1 is the case of an ideal Z-cut LiNbO3 modulator. Depending on the design of the electrodes, it may be 0.7 to 1, or −0.7 to −1. However, since there is a similar effect in improvement of waveform deterioration due to the SPM-GVD effect, for simplicity, in the following discussion, α=1 or −1. To be specific, as the operating range of the intensity modulator, in the case where the slope of a trough to a crest of the photoelectric response characteristics is used, α=1, and conversely, in the case where the slope of a crest to a trough is used, α=−1. Moreover, in the cases where the duty cycles of the RZ signal are 33% and 67%, the chirp characteristics change for each single bit, which causes waveform deterioration. In the structure of the conventional optical transmitter as shown in FIG. 11 described previously, the duty cycle of the output light is fixed. Therefore, there is a problem in that it is difficult to satisfy, at the same time, the duty cycle and the chirp characteristics most suitable for realizing excellent transmission characteristics.
In Japanese Unexamined Patent Publication No. 10-51389, a technique is described in which the duty ratio of an output light is varied in regards to an optical transmitter of an NRZ (Non Return to Zero) modulation format. However, since the known technique is targeted at an NRZ modulation format, and is also aimed at making the duty ratios of the output light the same before and after chirp switching corresponding to the dispersion characteristics of a transmission path, even if such a technique is applied to an optical transmitter corresponding to a modulation format such as RZ-DQPSK or the like, it is difficult to limit waveform deterioration effectively due to the SPM-GVD effect described above.